The contractile properties of the myocardium are determined largely by the amount of calcium available to the contractile proteins. The plasma membrane and the membranes of the sarcoplasmic reticulum play a key role in regulating calcium movements into and within the myocardial cell. In the present studies, we will define cellular mechanisms whereby calcium movements through these membranes are regulated. The reaction mechanism of calcium transport through the sarcoplasmic reticulum will be studied by reconstituting a functional calcium pump from known membrane components. Further studies will be carried out on the mechanism of the regulation of the Ca2 ion- activated ATPase of cardiac sarcoplasmic reticulum by cyclic AMP-dependent protein kinase. The effects of protein kinase. The effects of protein kinase-catalyzed phosphorylation of cardiac SR are reversed by dephosphorylation catalyzed by phosphoprotein phosphatase. This enzyme will be purified from cardiac sarcoplasmic reticulum and charaterized. It is hoped that the basic new information that will become available as a result of these studies will be useful in the development of new drugs and clinical procedures used in the treatment of patients with heart disease. Bibliographic refereces: Tada, M., M.A. Kirchberger, H.-C. Li and A.M. Katz. Interrelationships between calcium and cyclic AMP in the mammalian heart. In Calcium Transport in Contraction and Secretion (E. Carafoli, F. Clementi and A. Margreth, eds.), North-Holland Publishing Co., Amsterdam, The Netherlands, 1975, pp.373-381.